The present invention relates to a vacuum switching tube having a stationary contact and a contact which is arranged movably relative thereto in a straight line, as well as having a coil carrying the current to be switched, for generating a magnetic field permeating the space between the contacts.
A vacuum switching tube of this type has become known through DE-OS No. 30 33 632. In that switching tube, the coil has the purpose of generating an axially aligned magnetic field, i.e., a magnetic field, the lines of which extend substantially parallel to the longitudinal axis of the contact arrangement. Thus, the field lines extend substantially also parallel to the current filaments of the electric arc discharge occurring between the contacts, whereby a contraction of the arc is prevented in a manner known per se when large currents are switched. If it is desired to control switching currents of 30,000 amperes and more, the required magnetic field can be generated economically only if the coil used for this purpose is arranged in the immediate vicinity of the contacts or is formed by them themselves.
In the vacuum switching tube according to DE-OS No. 30 33 632 mentioned above, the coil generating the magnetic field is associated with the stationary contact. The desired axial field pattern is obtained by providing the contacts with ferromagnetic parts which allow the field lines to pass approximately axially into the space between the contacts.
The existence of the ferromagnetic parts at the movable contact of the vacuum switching tube described above makes it difficult to control a large switching current as well as a large permanent current. Because of the relatively low electric conductivity of the ferromagnetic parts, it is necessary to design the movable contact and its current lead-in bolt with an enlarged cross section in order to keep the temperature rise in continuous operation low, particularly with a permanent current of 3,000 A and more. However, the contact is given thereby a relatively large mass which has an unfavorable influence on the magnitude of the mechanical drive energy during the switching. In addition, the ferromagnetic parts allow a residual magnetic field to remain at the time of the zero crossing of the current, which makes it more difficult to quench the arc.